In low pressure discharge lamps, mercury dosed to the fill gas of the lamp is used for light generation. In order to produce visible light, phosphor coating is provided on the interior surface of the discharge envelope. Lamp manufacturers attempt to set the partial pressure of mercury vapor according to the highest radiation resonance line in order to accomplish the highest luminous output at the voltage and current density applied to the electrodes of the discharge envelope and to stabilize this partial pressure, as well. The adjustment and stabilization of mercury vapor partial pressure at a configuration of low pressure discharge lamps known so far is feasible by setting the temperature of a specially formed cold spot of the discharge envelope which is the coldest point of the lamp.
A helical configuration of low pressure discharge lamp is described in a patent issued in the late German Democratic Republic with No. 212 843. The discharge envelope of this lamp comprises a linear tube section and a helical tube section wound around the linear section with at least one loop. The linear section is connected to the helical section through a bridging at the upper end of the envelope further off the base of the lamp. This configuration of compact fluorescent lamp did not become a practical application because there would have been a need for two production lines for manufacturing the two different tube sections, which would have increased the expenses.
Formation of the cold spot is also difficult at this configuration since this can be accomplished only by means of lengthening the upper sealed part of the linear tube section in the vicinity of the bridging which results in poor esthetical appearance. The part of the helical section which connects to the linear section through the bridging has to be bent under a relatively sharp angle so that it can be connected to the linear tube section positioned in the central axis of the lamp. This sharp angle bending might cause peeling of the phosphor coating on the inner curve and cracked phosphor coating on the outer curve of the bent tube section which throw difficulties in the way of faultless manufacturing.
A double helix configuration of a low pressure discharge lamp is described in German Offenlegungsschrift DE 41 33 077 A1. The discharge envelope of the discharge lamp is helical in shape. At the top of the envelope, the ends of the tube sections are bent towards the central axis of the helix and the ends are melted together through a joining section of enlarged diameter. This enlarged diameter section is the cold spot of the lamp. When manufacturing this tube configuration, there is no need for two bending machines since the helical sections to be melted together are of identical shape. Nevertheless, bending under a relatively sharp angle makes faultless manufacturing also difficult due to the peeling and cracking of the phosphor coating.
U.S. Pat. No. 5,680,005 describes also a low pressure discharge lamp with a double helix configuration. The joining section of the two helical parts is formed of a slightly expanded linear tube that joins to the helically shaped tube parts through 180° bending. At this configuration of low pressure discharge lamp, the expanded linear tube can be considered as cold spot. However, the tube diameter in the 180° bending is reduced, thus the discharge arc warms up the walls of the tube, and the generated heat is conducted to the linear section making its operation as cold spot difficult. Due to the indefinite cold spot, the partial pressure of mercury vapor cannot be set to the optimum level and the luminous output of the discharge lamp does not attain the highest possible value.
Thus there is a particular need for a low pressure discharge lamp with a double helix shape which has a well-defined cold spot, a good esthetic appearance, and the manufacturing process of which is conducive to the faultless production.